The present invention relates to an apparatus for air separation utilizing liquefied natural gas (LNG) as a cold source. Specifically, the invention relates to an air separation apparatus suitable for supplying high-pressure nitrogen gas and oxygen gas to an integrated gasifier combined cycle plant.
Since LNG is consumed in a gas phase in a final stage, various kinds of processes have been proposed wherein cold generated during gasification of LNG is effectively used. For example, there is known a process utilizing LNG as a cold source in an air separation apparatus for producing nitrogen and oxygen by cryogenic separation.
Jpn. Pat. Appln. KOKOKU Publication No. 49-45054, for instance, discloses a process in which cold is utilized for directly cooling feed air. Jpn. Pat. Appln. KOKOKU Publication No. 52-41224 discloses a process wherein cold of LNG is utilized for cooling and liquefying nitrogen, which is restored to room temperature, compressed and recycled. Further, Jpn. Pat. Appln. KOKOKU Publication No. 46-16081 describes a process wherein cold of LNG is utilized directly for cooling both nitrogen, which is to be recycled, and feed air.
In a case where a nitrogen cycle is adopted in order to supply cold needed in an air separation process, cold is produced by compression, liquefaction and expansion of nitrogen. In this case, it is known that less power is consumed when nitrogen gas at a low temperature is compressed, than when nitrogen gas at room temperature is compressed. For this reason, of the above publications, Jpn. Pat. Appln. KOKOKU Publication No. 46-16081 adopts a process wherein nitrogen gas is cooled by LNG (or a low-temperature gas separated in an air separation apparatus) and then compressed, thereby reducing power cost. In the process described in this publication, however, nitrogen is recovered in a liquid phase by compression and liquefaction of separated nitrogen gas.
In an integrated gasifier combined cycle plant to which special attention has recently been paid, a large amount of high-pressure nitrogen gas and oxygen gas is consumed. On the other hand, the pressure of nitrogen gas or oxygen gas produced by a conventional air separation apparatus was insufficient. Thus, in general, such gases were compressed using a compressor at room temperature, and then supplied to the plant. Therefore, power cost was high for the operation of the compressor. Under the circumstances, an improvement was desired for reducing power cost in an integrated gasifier combined cycle plant.
On the other hand, cold of LNG is utilized in conventional air separation apparatuses in order to supply cold needed for air separation or to supply separated products to the outside in a liquid phase. However, there was no idea that cold of LNG is utilized for the purpose of increasing the pressure of a product gas.